The present invention relates to a distillation apparatus for the evaporation of a liquid and for its subsequent condensation, the apparatus comprising a plurality of flat, bag-like elements of a thin film material, such as plastic film, placed one against the other, the elements serving as heat exchangers between a vaporizing liquid which flows along the exterior surfaces of the elements and a condensing vapor which has been directed to the inside of the elements, and a compressor for increasing the pressure and temperature of the generated vapor before it is directed to the inside of the elements.
A distillation apparatus according to the above definition is described in, for example, FI Lay-Open Print 79948 and in the corresponding International Application Publication WO 90/01977. The primary use of distillation apparatuses of this type has been the production of fresh water from sea water. The tube or plate heat exchangers used in the prior art for the distillation of sea water were susceptible to contamination, and the corrosive nature of sea water caused problems of corrosion in them, necessitating the use of expensive, noncorrodible materials such as titanium and cupro-nickel. By the use of bag-like distillation elements made of plastic film these disadvantages have been eliminated, since plastic film is inexpensive, non-corrodible and, owing to its resilience, less susceptible to contamination. Any contaminant possibly adhering to the membranes can be shaken off by varying the pressure prevailing inside the elements. It has been possible to compensate for the poor thermal conductivity of plastic per se by using in the distillation apparatus a very thin film and a large number of thin elements having a large heat exchange surface.
The distillation apparatus according to said FI Lay-Open Print 79948 comprises, above the plastic membrane elements, a common distribution basin for the water to be evaporated, from which the liquid flows via pipes into the passages between the elements. The vapor generated on the exterior surfaces of the elements is directed to a blower, which blows it, at a higher pressure and temperature, to the inside of the elements through apertures in their sides. The system described has the disadvantage that, by means of it, the vapor to be condensed cannot be spread very evenly inside the elements, and subsequently part of the potential condensing efficiency of the apparatus remains unexploited. Also, in the apparatus disclosed, the spreading out of the liquid to be evaporated over the exterior surfaces of the elements does not occur in the best possible manner. Thus, heat exchange between the liquid vaporizing on the exterior surfaces of the elements and the vapor condensing inside the elements remains incomplete.